1. Field of the Invention
The present invention relates to ceramic flow tubes for use in magnetic flowmeters including regions of different ceramic composition to provide enhanced properties to form fluid contacts or magnetic pole pieces for the flowmeter.
2. Description of the Prior Art
U.S. Pat. No. 3,750,468 shows a flowmeter that has a ceramic liner.
Similarly, U.S. Pat. No. 4,507,975 shows a sensor for a magnetic flowmeter having an oxide ceramic measuring tube, with the measuring electrodes comprising metal inserts that are sintered into the ceramic tube during the time that the tube is being sintered. This forms a composite structure (metal-ceramic) with its inherent problems of bonding, weak zones, and possible cracks under highly pressurized fluids. The electrodes form stress concentrators, as well as areas of reduced corrosion and abrasion resistance. Also, the electrodes form protruding surfaces, which tend to reduce performance because of suspended particles hitting such electrodes. Sealing is also difficult, as the coefficients of thermal expansion of the ceramic and electrode materials are generally different.
Bonding of two different ceramic materials together to obtain special properties is disclosed in both U.S. Pat. Nos. 3,324,543 and 4,442,375. These patents also discuss the problems involved in obtaining metal to ceramic seals.
U.S. Pat. No. 4,388,834 shows an electromagnetic flowmeter that has a pair of electrodes of electrically-conductive synthetic plastic material fused in the liner.
U.S. Pat. No. 4,499,754, issued Feb. 9, 1985 shows an electromagnetic flowmeter that uses an insulating liner for lining the measuring pipe, with conductive powder forming conductive paths in an insulating material.